The Original HIIT, or High Intensity Interval Training, is a style of training that is raved about by many. It consists of doing one set of each exercise, but to complete failure. It has desirable effects on many that are short for time and ones that have not been in the gym for years. There are many mixed reviews about this training protocol, many of the reviews stem from the bodybuilding world where many are seeing great results. The results are not coming from the program itself but rather from the darker side of bodybuilding, drug use. Popular steroids increase the androgen receptor content of cells, this is the same effect that one gets when they work out. However, our receptors only increase on the muscles that we are working at that given time. So if I was to do chest, then I would really only be increasing receptor content on my chest muscles.
The increased receptor count on the cells allows for increased hormone interaction with the cells. As you exercise you increase blood flow to the worked muscles. The hormones that are released during exercise and naturally throughout the day then interact with the androgen receptors of the muscle cells and elicit cellular responses. With steroid users the hormones are already in the body and do not need to be released with different workout strategies AND the receptor content of the muscles is already increased leaving blood flow to be the key factor. With HIIT training, the blood "pump" is very profound and is desirable to increase hormone and receptor interaction time.
For the average gym and fitness enthusiast we have a much harder task, and must release hormones through varying weight, reps, sets, and rest during our workouts and then eliciting the "pump". In order to get full response from all muscle fibers high and low weights must be used to increase motor unit recruitment. This is referred to as The Size Principle. In order to elicit the greatest cellular response you must target each fiber and use Time Under Tension.